I used to have a default setting of three cells, when using math (cmd+m, enter). But at some point it has changed to only to cells, and when I add an additional colon, when there are not aligned.
I have tried to uninstall, reinstall and change version of LyX. I run Mactex and then Lyx on a MacBook Air.
eqnarrayenvironment for the default aligned, display math. That is a bad idea (https://tex.stackexchange.com/questions/196), so at some point this was changed to using analignenvironment. Aneqnarraywill have three columns per alignment, with the equality sign (or similar) in the middle column, butalignwill have two columns per alignment, and the equality sign should go in the right column. – Torbjørn T. Nov 14 '19 at 16:02